1. Field of the Invention
The present invention relates to a semiconductor vertical-cavity, surface-emitting laser, and, more particularly, to a semiconductor vertical-cavity, surface-emitting laser having an expanded cavity to increase the power of the emitted beam of light while maintaining the beam in a single transverse mode.
2. Description of the Prior Art
Semiconductor laser diodes, in general, comprise a body of a semiconductor material having adjacent regions of opposite conductivity type forming a p-n junction therebetween. The body is adapted to generate and emit radiation when an appropriate potential is applied across the p-n junction. Vertical-cavity, surface-emitting lasers (VCSELs) emit radiation in a direction perpendicular to the plane of the p-n junction or substrate rather than parallel to the plane of the p-n junction as in the case of conventional edge-emitting diode lasers. In contrast to the astigmatic beam quality of conventional edge emitting lasers, VCSELs advantageously emit a circularly symmetric Gaussian beam and thus do not require anamorphic correction. VCSELs, moreover, can readily be made into two-dimensional laser arrays as well as be fabricated in extremely small sizes. Accordingly, two-dimensional VCSEL arrays have various applications in the fields of optical interconnection, integrated optoelectronic circuits and optical computing, optical memory, optical communications, laser printing, scanning, etc.
To achieve a low threshold current, VCSELs typically utilize a thin active region on the order of .lambda./4n thick or less, where .lambda. is the wavelength of the emitted light and n is the index of refraction of the active region. With such a thin active region, however, VCSELs have a single pass optical gain of approximately 1% or less, thereby requiring the use of end mirrors having reflectivities greater than 99% to achieve lasing. Such a high reflectivity is normally achieved by employing epitaxially grown semiconductor distributed Bragg reflector (DBR) mirrors. Also, mirrors have been made employing multilayers of dielectric materials. Penetration of the light intensity into multilayer mirrors gives rise to an effective optical cavity length which is somewhat longer than the length of the material between the mirrors. Much of the prior art describes VCSELs with effective optical cavity length not much larger than the optical thickness of the active material, and in all cases the ratio of the effective optical cavity length to the active material thickness of the active material, and in all cases the ratio of the effective optical cavity length to the active material optical thickness is less than 100. Optical lengths are well known as being the physical length multiplied by the refractive index of the material. For the case of heterostructures, the optical length is the sum of the products of physical lengths times refractive indices. A precise and general technique for evaluating the effective optical cavity length is described in an article by J. Jewell et al., entitled "High-finesse (Al,Ga) As interference filters grown by molecular beam epitaxy." published in Applied Physics Letters, Vol. 53(8), Aug. 22, 1988, pgs. 640-642. The optical thickness of the active material in the cases of bulk or superlattice semiconductors, is straightforwardly the refractive index multiplied by the physical thickness. In the case of quantum well active material, the physical thickness does not include the barrier material between the wells. Penetration of the light intensity into the multilayer mirrors can be increased, thereby increasing the effective optical cavity length, by choosing mirrors layers whose differences in indices of refraction are smaller. Another means for increasing the penetration of light intensity into the mirrors, thereby increasing the effective optical cavity length, is to introduce materials having intermediate indices or refraction between the layers of high and low index of refraction. A laser having both a semiconductor mirror and a dielectric mirror is described in an article by K. Mori et al, entitled "Effect of cavity size on lasing characteristics of a distributed Bragg reflector-surface emitting laser with buried heterostructure", published in Applied Physics Letters, Vol. 60(1), Jan. 6, 1992, pgs. 21-22.
Unfortunately, the applicability of vertical-cavity, surface-emitting lasers is severely limited by its low power output. Single transverse mode emission at current significantly above threshold, e.g.&gt;20 milliamps, for beams having a diameter well below 10 micrometers have been reported. However, attempts to provide a beam of greater diameter have resulted in a laser going multimode at low currents, e.g.&lt;15 milliamps. Therefore, it would be desirable to have a vertical-cavity, surface-emitting laser which emits a large diameter beam in a low-order transverse mode to provide a beam having increased power.